Energy-dispersive radiation spectrometry systems, such as, without limitation, X-ray spectrometry systems or gamma-ray spectrometry systems, are used for detecting, measuring and analyzing radiation emissions, such as X-ray emissions or gamma-ray emissions, from, for example, a scanning electron microscope (SEM). A typical energy-dispersive radiation spectrometry system includes the following four main components: (1) a detector, (2) a pre-amplifier, (3) a pulse processor, and (4) a computer-based analyzer. For convenience only, and not for purposes of limitation, the following description will relate to X-ray spectrometry systems and photons in the form of X-rays (as compared to, for example, photons in the form of gamma-rays that are detected in a gamma-ray spectrometry system).
The detector, which usually takes the form of a semiconductor sensor of some type, converts an incoming X-ray into a very small current pulse, typically on the order of tens of thousands of electrons, with a duration of about tens to a few hundreds of nanoseconds. The magnitude of each of the current pulses is proportional to the energy of the X-ray.
The pre-amplifier amplifies the current pulse output by the detector and typically converts it into a voltage signal in the range of tenths of millivolts up to a few hundreds of millivolts. There are two main types of preamplifiers: “tail pulse” or RC-coupled preamplifiers, and pulsed-reset preamplifiers. The subject matter described elsewhere herein applies to both types of preamplifiers. However, for convenience, the subject matter is described with reference to the pulsed-reset type.
In a pulsed-reset type of preamplifier, the charge generated in the sensor is integrated in a feedback capacitor such that the resulting voltage increases in steps of varying heights and intervals, until it reaches an upper limit. When that limit is reached, a “reset” pulse is applied which drains the accumulated charge from the feedback capacitor, restoring the preamplifier to near its minimum output voltage in a short time, typically a few microseconds. Then, charge due to the interaction of X-rays with the detector accumulates on the feedback capacitor again, and the cycle repeats. The output signal of a pulsed-reset preamplifier is known in the art as a “ramp”, due to its characteristic when observed with an oscilloscope of having a slow, irregular rise followed by a rapid return to its lower limit. In contrast, tail-pulse preamplifiers act as high-pass filters on the voltage step signal output by the detector, with an exponential return to baseline whose time constant is long compared to the charge integration time in a feedback capacitor of the preamplifier.
The pulse processor receives the pre-amplifier signal and generates a numeric representation of the X-ray's energy through an integration process. In older energy-dispersive radiation spectrometry systems, the pulse processor included two separate components, namely a “shaping amplifier” and an analog to digital converter. Modern energy-dispersive radiation spectrometry systems, on the other hand, typically combine these functions, with the newest designs digitizing the preamplifier signal directly and carrying out all pulse detection and filtering functions using digital signal processing.
The computer-based analyzer accumulates the X-ray energies output by the pulse processor into a spectrum or plot of the number of X-rays detected against their energies. The spectrum is divided into a somewhat arbitrary number of small ranges called “channels” or “bins.” In older systems, a hardware component called a multi-channel analyzer (MCA) did the accumulation of X-rays into spectrum channels and a computer read out the summed result. In modern systems, the MCA function is handled in software, either by the computer or even within the pulse processor.
The job of the pulse processor is made more complex by several factors. For example, electronic noise is superimposed on the underlying signal received from the preamplifier. For X-rays that are near the lowest detectable energy level, the preamplifier output step height may be significantly smaller than the peak-to-peak excursions of the electronic noise. In such as case, the X-ray can only be detected by filtering the signal for a relatively long period of time before and after the step, to average away the contribution of the noise. The amount of such noise averaging is a fundamental operating parameter of all pulse processors. This averaging time is variously referred to in the art as “shaping time” or “peaking time.”
Resolution of detectors for X-ray microanalysis is customarily reported as the full width in electron volts (eV) at half the maximum peak height (FWHM) of the Mn K-alpha emission line. Good quality lithium drifted silicon (Si(Li)) detectors have an optimum peaking time of up to 80-120 microseconds (μS) achieving a resolution under 130 eV with a maximum counting rate of around 1-2 kilocounts/sec (kcps), and a usable minimum peaking time around 2 μS at a resolution of perhaps 240 eV with a maximum counting rate around 50 kcps. The majority of X-ray microanalyzers currently installed on scanning electron microscopes (SEM) use Si(Li) detectors.
Recently, a new generation of X-ray sensors known as silicon drift detectors (SDDs) has come on the market, with very different characteristics in several respects. In SDDs, the device capacitance is several orders of magnitude lower than in Si(Li) detectors because the electrons generated when an X-ray strikes the sensor are guided by the internal bias field to a small-spot anode. As a result, the rise time of the preamplifier output signal is up to four or five times shorter than for Si(Li) detectors, and the peaking time for equivalent energy resolution is enormously reduced, from 80-100+ μS down to perhaps 2-4 μS. Also, an SDD operates at a higher temperature, and therefore higher leakage current, than a Si(Li) detector. Leakage current noise and various series and parallel noise components cause the measured resolution for an SDD as a function of peaking time to go through an optimum, then begin to become worse as the peaking time is increased beyond the optimum.
Because the peaking time in SDDs is reduced by a factor of about 25-40, the potential maximum counting rates are correspondingly higher. The superior rise time of SDDs allows for better correction at these high count rates of an error known in the art as “pulse pile-up”. This occurs when two X-rays arrive at the detector so close in time that they cannot be distinguished from a single X-ray at the sum of their energies. Pulse processors specifically designed to take advantage of the characteristics of SDD detectors to improve pile-up detection are just now coming on the market.
Existing X-ray spectrometry systems, and in particular the pulse processors thereof, were designed around the characteristics of the Si(Li) detector which has been in use for about 40 years. As a result, existing pulse processors are not well suited to handle the output of SDDs (and associated preamplifiers). Specifically, in some cases, the existing pulse processor cannot be set to a sufficiently short peaking time to give optimum resolution from an SDD detector. The noise of a silicon detector becomes worse if the peaking time is longer than optimum, due to shaping-time-dependent noise as described above. This is rarely an issue with Si(Li) detectors, whose optimum shaping time is very long, but can become a problem with SDDs. A commercial example of a pulse processor which cannot be configured optimally for an SDD is the Gresham Titan model analog processor, which only has shaping time settings of 10 μS and 40 μS, while the optimum for an SDD is typically around 2-4 μS.
With the benefits of SDDs described above, users of such systems will likely want to replace the Si(Li) detectors (and associated preamplifiers) with SDDs (and associated preamplifiers). Because of the limitations of existing pulse processors described above, users of existing X-ray spectrometry systems wishing to switch to SDDs will also need to employ a new pulse processor designed to function with SDDs. From the user's point of view, it is desirable to not have to modify the existing X-ray spectrometry software to support the new pulse processor directly for a number of reasons. First, many of these existing systems are no longer well supported by the original manufacturers, due to a series of mergers and acquisitions over the last 10-15 years, or would require costly purchase of software upgrades. In addition, purchase of an entire new spectrometry system for the SDD would be significantly more expensive than purchasing only the SDD and a new pulse processor. Finally, it would also require the user to learn an entire new suite of spectrometry software.
U.S. Pat. No. 6,369,393 to Jordanov uses (possibly replicated) digital samples which have noise characteristics determined by the first stage of the preamplifier to fill in the required time separation between events before pulse shaping. The subject mater described herein contemplates improving performance by pulse shaping in one pulse processor to measure the X-ray energies, and then separating edges reconstructed from those energies by a signal with much lower noise than the first preamplifier stage for processing by a second pulse processor.